Conversion coating of magnesium alloy surfaces



United States Patent 3,278,343 CUNVERSION COATING 0F MAGNESIUM ALLOYSURFACES John M. Leuzinger, Detroit, Mich., assignor to Amchem Products,llnc., Ambler, Pa., a corporation of Delaware No Drawing. Filed Mar. 12,1963, Ser. No. 264,660 8 Claims. (Cl. 1486.16)

This invention relates to the art of coating magnesium and is moreparticularly concerned with the application of chemical conversioncoatings on magnesium alloy surfaces. It provides a new method forforming such coatings, which method is especially useful in the coatingof certain magnesium alloys which are unusually diflicult to coat. Forinstance, alloys of magnesium which contain from about 6 to about 9% ofaluminum are the alloys to which the invention is particularlyapplicable. Examples of alloys of this kind are the so-called diecastingalloys and those types which are referred to as sand mold'castings andextrusions. These alloys, in addition to their 69% of aluminum content,also contain from about 0.5 to about 3% zinc and from about 0.13 toabout 0.3% manganese, the balance being magnesium. Typical, butnon-limiting, examples of alloys of this type are those which aredesignated by the American Society of Testing Materials as AZ-63, AZ-91and AZ92.

Before proceeding to a detailed description of the present invention itis desired to refer briefly to certain facts which are well known tothose skilled in this art. Magnesium, of course, is relatively reactiveand comparatively easy to coat but despite this face very few, if any,of the coating procedures heretofore advanced for the purpose have beencompletely acceptable, especially from a practical commercialstandpoint. This applies particularly to the alloys above referred to. Afew processes have been adopted for commercial use but they involverather long, laborious treatments which are expensive to operate and arewholly inconsistent with the rapid, present day production techniqueswhich are demanded by industry.

Among prior processes for the coating of magnesium surfaces are thosewhich combine dichromates with either nitrates or fluorides; orchromates in combination with chlorides or mixtures of chlorides andnitrates; or mixtures of chromates and phosphates. Furthermore, thecoating processes which utilize halides, particularly chlorides, havenot proven satisfactory in view of the corrosive action which isimparted to the surface by the halide ions occluded in the coatings.

With all of the foregoing in mind, the principal object of the presentinvention is to provide an improved method of forming chemicalconversion coatings on the surface of magnesium alloys, which methodcompletely overcomes the objections of former practices, which yieldscoatings having excellent paint bonding and corrosion resistantproperties, which can be applied in very short periods of time, whichcan be employed at very much less cost and which is particularly usefulin the coating of the alloys mentioned above.

How the foregoing objects and advantages together with such others asare incident to my invention or which may appear hereinafter areattained will now be described in detail.

Broadly speaking, I have discovered that if clean magnesium alloysurfaces, and especially the surfaces of alloys of the type describedabove, are subjected first to the action of an aqueous activatingsolution consisting essentially of a phosphate having a molar ratio ofcationic oxides to anionic oxides of from 1 to 2 plus fluoride ions inamounts as hereinafter specified and, following water rinsing, arethereafter subjected to the action of a halidefree coating solutioncontaining, as its principal and es sential coating producingingredients, the ions of both 3,278,343 Patented Oct. 11, 1966 trivalentand hexavalent chromium plus a phosphate, in the amounts and ratios asspecified hereinafter, there is produced on the surface a chemicalconversion coating which has unusually excellent paint bonding andcorrosion resistant properties.

As noted above, the aqueous activating solution must contain a phosphatehaving a molar ratio of cationic oxides to anionic oxides of from 1 to2. Such phosphates as defined in the Encyclopedia of ChemicalTechnology, volume X, pages 403-405 (1953), include metaphosphates,wherein the cationic to anionic oxide ratio is 1, polyph'osphates,having ratios between 1 and 2, and pyrophosphates, wherein the ratio is2. While all of these phosphates have been found to performsatisfactorily in the activator solutions of this invention, thepreferred phosphates have been found to be the pyrophosphates inasmuchas these provide greater heat stability with less reversion to the orthoform than either the meta or polyphosphates.

The amount of phosphate, from the class described, which must beemployed in the aqueous activator solutions of this invention has beenfound to be from 4 to 70 grams/liter, calculated as P 0 So far as thefluoride ion content is concerned, it has been found that thisconstituent must be utilized in amounts of from 0.5 to 10 grams/liter,calculated as F, 1n order to insure obtaining the improved coatings onmagnesium die-casting alloy surfaces.

In addition to maintaining the phosphate and fluoride ion content withinthe limits noted hereinabove, it is also essential that the solution pHbe maintained between 7.0 and 11.0 in order to insure obtaining therequired activation of magnesium alloy surfaces prior to coatingformation. Where the activator solution pH is allowed to fall below theminimum pH value of 7.0, or to rise above the 11.0 maximum limit, thesubsequently produced coat- 1ngs on magnesium surfaces will be found tobe thin and uneven and to possess little or no corrosion resistance.

So far as introduction of the phosphate and fluoride ions are concernedit is preferred to use tetrasodium, potassium or ammonium salts of thephosphate acids, particularly of pyrophosphoric acid, and the sodium,potassium or ammonium fluoride salts. Use of these salts providessolution pHs within the range desired with no need for subsequentadjustment thereof. However, it is within the purview of this inventionto employ any of the alkali metal salts of these phosphate acids, or toemploy only phosphate acids and hydrofluoric acids in preparing theactivator solutions of this invention, provided of course that thesolution pH is adjusted to the range noted above. The solution pH iscontrolled by the use of alkali metal or ammonium hydroxides where lessbasic salts or the acids of the essential anions are employed.

In general, it has been found that where the amount of phosphate ionpresent is on the high side of the permitted range then the amount offluoride ion should be maintained near the upper permissible limit.Conversely, where the phosphate ion concentration is more dilute, thatis where it is present in the lower limit of concentration, then theamount of fluoride ion should also be kept on the low side of thepermitted range.

The temperature at which the activator solution is operated is criticalit completely satisfactory coatings are to be obtained. It has beenfound that the temperature range of to F. provides the requiredactivation of magnesium alloy surfaces. Where temperatures of less than130 F., or greater than 180 F. are employed the magnesium surfaces willnot properly be conditioned for reception of a chemical conversioncoating of the second stage, and subsequently produced coatings will notdemonstrate the required paint bonding and/or corrosion resistantqualities.

The overall time of treatment is not critical so long as the activatorsolution temperature is maintained between l30180 F. as notedhereinabove. However, it is preferred to operate the activator stage forperiods of time ranging from 30 seconds to about 5 minutes duration.

Insofar as the second major step is concerned, i.e. the coating step perse, the following factors are to be observed.

The coating solution must be prepared so as to contain, initially, atleast 0.75 grams/liter of trivalent chromium ion calculated as Cr Wherethe coating solutions are initially prepared without the addition ofthis essential ingredient, notwithstanding the fact that all of theother ingredients may have been added thereto, the solution simply willnot produce the required corrosion resistant and paint bonding coatingson magnesium die-casting alloy surfaces.

Use of large amounts of trivalent chromium, for example up to about 20grams/liter does not appear to cause any deleterious effects upon eitherthe coating formation or its quality. However, since no added benetfisare obtained from high concentrations of this constitutent it ispreferred to add no more than about 1.5 grams/liter of trivalentchromium to a freshly prepared coating solution.

Introduction of the trivalent chromium ion may be by way of any saltwhich is soluble in the coating solution, just so long as the anionthereof is in no way detrimental to the coating reaction. A preferredsalt is chromic nitrate since this is not only readily available, but italso aids in controlling the acidity of the coating solution.

Hexavalent chromium ion must be present in the coating solution inamounts ranging from 2 to 46 grams/liter (calculated as Cr). Where lessthan 2 grams/liter of hexavalent chromium are used, a soft,non-adherent, black coating is obtained; whereas use of more than 46grams/ liter of hexavalent chromium results in little or no coatingbeing formed upon magnesium die-casting alloy surfaces.

A preferred range of hexavalent chromium ion content has been found tobe from about 4 to 30 grams/liter, since within this narrower rangeoptimum coatings have been obtained consistent with desired quality.

The hexavalent chromium ion may be supplied by chromium trioxide, or byan alkali metal or ammonium chromate or dichromate salt. It is preferredto utilize chromium trioxide since this particular source of hexavalentchromium ion does not introduce alkaline cations which in turn requirethe addition of hydrogen ion in order to maintain the essential pH rangeas discussed hereinafter.

Phosphate ion (calculated as P is preferably introduced in the form ofphosphoric acid, and this is particularly true where a salt of chromicacid has been employed. If desired, however, the phosphate may be addedin the form of an alkali metal or ammonium salt, but such usage willnecessitate pH adjustments to compensate for the alkalinity of thecations employed. The amount of phosphate ion, calculated as P0 which isessential to the successful operation of the process of this inventionhas been found to be from about 3 to about 52 grams/liter of the coatingsolution. Where less than 3 grams of phosphate ion are employed nouseful coating is formed on the magnesium alloy surfaces, if indeed acoating is formed at all. Conversely, if more than about 52 grams ofphosphate ion are utilized the coating produced will be non-adherent andcompletely unsuited to receive a siccative finish.

Experience with the process of this invention has shown that use of from5 to about 44 grams/liter of phosphate ion, calculated as P0 providesexcellent and completely acceptable coatings, so that this narrowerrange of phosphate ion is preferred in order to insure optimum coatingresults.

In addition to the requirements that the hexavalent chromium andphosphate ion concentrations (calculated as Cr and P0 respectively) bemaintained within the limits indicated above, it is also essential thatthe ratio of hexavalent chromium to phosphate be maintained within therange of 1 part of hexavalent chromium to from 0.5 to 2.5 parts of P0This range has been found to be absolutely essential for the productionof corrosion resistant, .paint bonding coatings on magnesium die-castingalloys which contain from 6 to 9% aluminum, since coating solutions,even if prepared to contain the above indicated amounts of bothhexavalent chromium ion and phosphate ion, will fail to produce thedesired coating quality if the ratio of hexavalent chromium to P0 liesoutside the range of 1 to from 0.5 to 2.5 as indicated above.

The acidity of the coating solution is critical, and if completelysatisfactory results are to be obtained, such acidity, as measured bypH, must fall within the narrow range of 0.8 to 1.2. Where the solutionacidity is permitted to fall below the 0.8 lower limit the resultantcoatings will be soft and non-adherent. Conversely, use of a solutionhaving an acidity, as measured by pH, above 1.2 will result in darkcolored, soft and non-adherent coatings having essentially no paintbonding or corrosion resistant properties.

In the interests of insuring the desired high quality coatings whichresult from the process of this invention, it is preferred to operatethe coating solution within the narrower pH range of 0.9 to 1.1. Thispreferred range provides a small working margin on either side but stillwithin the broader critical pH range specified and serves to guardagainst obtaining poor, non-adherent coatings which will result, asnoted hereinabove, from operation outside the indicated critical pHrange.

In order to maintain the required pH range throughout operation of thecoating process it is periodically necessary to add increments of nitricacid, since neither phosp'horic nor chromic acids when used alone willyield a pH sufficiently low to fall within the required range. Use ofhalide ions, particularly chloride ions, in any form must be strictlyavoided in view of adverse corrosion results caused by halide ionsoccluded within the coatings produced upon magnesium alloy surfaces. Useof sulfuric acid for pH control must also be avoided since it has beenfound that the presence of sulfate ions leads to powdery coatingformation, which coatings fail to provide either adequate corrosionresistance or the desired paint bonding properties. Should the coatingsolution pH be found to be lower than 0.8, the addition of ammonium oralkali metal hydroxides will serve to restore the pH to the requiredrange.

The temperature at which the coating stage may be operated is notcritical and may range from average living room temperature (i.e. 70 F.)to as high as 190 F. However, the preferred operating range has beenfound to be from about 70 to about F. using a contact time of as littleas 20 seconds to several minutes. Generally, as is well known in theart, a lower coating temperature will require longer contact times,while the reverse is also true.

It is of course important that the magnesium die-casting alloy surfacesbe degreased or otherwise cleaned and pickled according to wellestablished prior art practices before treatment by the process of thisinvention. However, since neither the cleaning nor the pickling stagesform any part of the present invention, they are not described indetail.

The magnesium articles may be immersed in the treating solutions of thisinvention or the treating solutions may be sprayed, flowed, brushed orotherwise brought into contact with the magnesium alloy surfaces inaccordance with conventional practices in this art.

In order to demonstrate the process of the present invention, magnesiumalloy AZ-91 (9.0% aluminum, 0.2% manganese, 1.0% zinc, balancemagnesium) castings were cleaned and pickled according to well knownprior art practices and then subjected to the action of an activatorsolution containing:

Example I Ingredient: Grams N84Pz07 NaF 3 4 Water, to make 1 liter.

Ingredient: Grams NaH2PO4 8 Nagcrzoq CrO 5 .4 Cr (N0 3 6 Water, to make1 liter.

This solution was adjusted to a pH of 1.0 by adding thereto 14 grams of42 B HN0 per liter of solution.

The activated magnesium alloy castings were immersed in this coatingsolution for a period of 1.5 minutes at 85 F., and were thereafterrinsed with fresh Water. F 01- lowing this coating treatment thecastings were painted with a primer finish and baked at 250 F. for tenminutes. A baking enamel was then applied and cured at 250 F. for 20minutes.

The treated and enameled castings were then subjected to standard saltspray corrosion testing (ASTM B-1l7-57T) and after 1047 hours thesecastings were found to be perfect in their resistance to corrosion asmeasured by this test method.

Control castings of the same magnesium alloy similarly treated in theactivator and coating solutions, but unpainted, were found to show onlyvery slight corrosion after 1047 hours exposure in the salt spray test.

Additional castings treated by the above process and including bothpainted and unpainted specimens were subjected to standard water soakcorrosion testing in accordance with ASTM D-870-54T and after a periodof 912 hours were found to be in perfect condition. Still other AZ-91alloy magnesium die-castings, treated in accordance with the foregoingexample, and then painted, were subjected to standard humidity testingin accordance with ASTM JAN H-792 and after 1000 hours of exposure werefound to be in perfect condition.

Example II Magnesium AZ63 (6.0% aluminum, 0.25% manganese, 3.0% zinc,balance magnesium) die-casting alloys were immersed in a solutioncontaining:

Ingredients: Grams NflzHzPzOq 1 HF 0.53 Non-ionic wetting agent 0.1Water, to make 1 liter.

The pH of this solution was adjusted to with 50% NaOH solution. Acontact time of 2 minutes at 175 F. was employed after which thecastings were water rinsed and then subjected to the action of thefollowing coating Water, to make 1 liter.

This solution was adjusted to a pH of 0.9 by adding thereto 42 B. HNO Acontact time of 3 minutes at 75 F. was employed after which the castingswere immediately water rinsed. Following rinsing, both primer and bakingenamel finishes were applied to the castings as noted following ExampleI and various of the castings so treated were subjected to the standardsalt spray, water soak and humidity tests. All of the corrosion testresults showed perfect, unmarred surfaces after 1000 hours each in watersoak and humidity tests and after 2117 hours in salt spray, all testsbeing identical to the ASTM test methods listed after Example I.

Example III Magnesium AZ-92 (9.0% aluminum, 0.15% magnesium, 2.0% zinc,balance magnesium) alloy die-castings, previously cleaned and pickled inaccordance with the prior art procedures, were immersed in an activatingsolution containing the following constituents:

Ingredients: Grams N34P207 KF 30.5 Non-ionic wetting agent 0.1

Water, to make 1 liter.

The pH of this solution, as prepared, was 9.5. The alloy castings wereimmersed in this solution for 30 seconds at 180 F. After treatment thecastings were water rinsed and immediately subjected to the action ofthe following coating solution:

Ingredients: Grams (NH4)2CI'O4 C1'O3 Cr(NO 7.5 H PO 45 Water, to make 1liter.

The pH of this solution was 1.2 and the magnesium diecastings wereimmersed in the coating solution for a period of 25 seconds at 75 F.After coating the castings were water rinsed and then had appliedthereto both primer and baking enamels as indicated following Example Iabove. Standard water soak and humidity testing, in accordance with theforegoing examples, showed that after 1000 hours there was no corrosionon any of the diecasting alloy surfaces. After 2117 hours in 5% saltspray testing, examination of the castings showed only very slightcorrosion on the edges thereof. It will be noted that the activatorsolutions of Examples II and III incorporated therein a small quantityof a non-ionic wetting agent. It is within the purview of this inventionto add from 0.01 to 0.5% of a non-ionic wetting agent in order toincrease the wettability of the activator solution with respect to themagnesium alloy surfaces. Preferred nonionic Wetting agents include thepolyethoxylated alkyl phenols having from 6 to 15 mols of ethylene oxideand from 8 to 9 carbon atoms in the alkyl chain.

The foregoing results clearly demonstrate the very high degree ofcorrosion protection afforded to magnesium alloys containing 6 to 9%aluminum as well as the excellent paint adhesion qualities resultingdirectly from the process of this invention.

I claim:

1. In the art of applying a chemical conversion coating to a magnesiumalloy surface of the type which contains from approximately 6 toapproximately 9% of aluminum, the method which comprises (A) subjectingthe surface to the action of an aqueous activating solution consistingessentially of a phosphate having a molar ratio of cationic oxides toanionic oxides of from 1 to 2, the quantity of such phosphate being from4 to 70 grams/ liter, calculated as P 0 and fluoride ions in an amountof from 0.5 to 10 grams/liter, calculated as F, the pH of saidactivating solution being between 7.0 and 11.0 and its temperaturebetween and F.;

(B) rinsing the surface; and then (C) subjecting the surface to theaction of a coating solution consisting essentially of trivalentchromium ion, hexavalent chromium ion and phosphate ion the quantity oftrivalent chromium ion being at least 0.75 gram/liter, of hexavalentchromium ion from 2 to 46 grams/liter (calculated as Cr) and ofphosphate ion (calculated as P from about 3 to about 5 52 grams/liter;maintaining the ratio of hexavalent chromium to phosphate within therange of 1 part of hexavalent chromium to from 0.5 to 2.5 parts of P0and maintaining the acidity of said coating solu tion (as measured bypH) within the range of 0.8 to 1.2.

2. The method of claim 1 wherein the phosphate for the activatingsolution is chosen from the class which consists of the pyrophosphates.

3. The method of claim 1 wherein the phosphate for the activatingsolution is chosen from the class which consists of the tetrasodium,potassium and ammonium salts of the phosphate acids and, further,wherein the fluoride is chosen from the class which consists of thesodium, potassium and ammonium fluoride salts.

4. The method of claim 1 wherein the pH of the coating solution ismaintained at from 0.9 to 1.1 by adding nitric acid as required.

5. The method of claim 1 wherein the temperature of the coating solutionis maintained at from to F 6. The method of claim 1 wherein thetrivalent chromium ion is supplied by employing chromic nitrate.

7. The method of claim 1 wherein the hexavalent chromium ion is suppliedby employing chromium trioxide.

8. The method of claim 1 wherein the hexavalent chromium ion is suppliedby employing chromium trioxide in an amount of from 4 to 30 grams/liter.

References Cited by the Examiner UNITED STATES PATENTS 1/1959 Dell148-6.l6 2/1964 De Long et a1 148-6.27 X

1. IN THE ART OF APPLYING A CHEMICAL CONVERSION COATING TO A MAGNESIUMALLOY SURFACE OF THE TYPE WHICH CONTAINS FROM APPROXIMATELY 6 TOAPPROXIMATELY 9% OF ALUMINUM, THE METHOD WHICH COMPRISES (A) SUBJECTINGTHE SURFACE TO THE ACTION OF AN AQUEOUS ACTIVATING SOLUTION CONSISTINGESSENTIALLY OF A PHOSPHATE HAVING A MOLAR RATIO OF CATIONIC OXIDES TOANIONIC OXIDES OF FROM 1 TO 2, THE QUANTITY OF SUCH PHOSPHATE BEING FROM4 TO 70 GRAMS/LITER, CALCULATED AS P2O7, AND FLUORIDE IONS IN AN AMOUNTOF FROM 0.5 TO 10 GRAMS/LITER, CALCULATED AS F, THE PH OF SAIDACTIVATING SOLUTION BETWEEN 7.0 AND 11.0 AND ITS TEMPERATURE BETWEEN130* AND 180* F.; (B) RINSING THE SURFACE; AND THEN (C) SUBJECTING THESURFACE TO THE ACTION OF A COATING SOLUTION CONSISTING ESSENTIALLY OFTRIVALENT CHROMIUM ION, HEXAVALENT CHROMIUM ION AND PHOSPHATE ION THEQUANTITY OF TRIVALENT CHROMIUM ION BEING AT LEAST 0.75 GRAM/LITER, OFHEXAVALENT CHROMIUM ION FROM 2 TO 46 GRAMS/LITER (CALCULATED AS CR6+)AND OF PHOSPHATE ION (CALCULATED AS PO4) FROM ABOUT 3 TO ABOUT 52GRAMS/LITER; MAINTAINING THE RATIO OF HEXAVELENT CHROMIUM TO PHOSPHATEWITHIN THE RANGE OF 1 PART OF HEXAVALENT CHROMIUM TO FROM 0.5 TO 2.5PARTS OF PO4; AND MAINTAINING THE ACIDITY OF SAID COATING SOLUTION (ASMEASURED BY PH) WITHIN THE RANGE OF 0.8 TO 1.2.